In this blog I'm advocating an MMORPG where all players advance 24/7, and can invest their progress in any area of advancement they wish, even "working at a job" to make money based on other skils they might have. When they create an avatar to start the game, they get to distribute their initial character stats as they wish. When advancing, one can set their advancement to accrue in any area they wish.
Since this generates a population without any categorizable features even at the most basic level, obviously one cannot generate an automatic "con" system, where a player can immediately see if they have a good chance of defeating a mob or not. However, since there is no death penalty in the game, there is no reason to not simply try.
One must remember that in this true sandbox game, there are more means of "defeating" mobs than just killing them. One can become an advanced animal trainer and tame many mobs (or at least pacify them in order to get by them); one can also become an expert diplomat, learn the language of the mob, and negotiate for whatever they want; one can seduce or confuse the mob, or become an expert at camoflage and disguise to fool the mob.
However, in just fighting a mob, one can invest some advancement time in learning about that particular mob and get a detailed spec sheet about range of hit points, offensive capability, defensive, special attributes, drops, aggro, race affiliations, habits, treaties, known affililates and opportunities, language, range, food, etc. Having such information will allow the player to tailor specific training strategies that will advance particular traits, abilities, talents, and skills in order to be able to defeat that particular mob in combat.
In this way, if a particular crafting skill you have requires, say, the fur off of a snow leopard, you could set your avatar to work to earn money to buy the furs you require from other players, or you could go in and tame a leopard, bring it to town and let your stronger friends kill it for you so you can then get the fur; you could tame a creature that can kill a snow leopard, take it with you and get your furs that way; you could do some work for a neighboriing tribe and take the trade in furs; or, you could examine detailed information about the snow leopards and train yourself up in the appropriate commodities so that you can defeat it in combat yourself and then skin it (and perhaps process it for other valuable resources).
So no, you will not be able to immediately "con" a mob and know if you can defeat it, but you can spend a short time learning about the mob and from that info figure out how you can either defeat it or get what you want from it.